Chute Design For Belt Cleaning
Brian Merchant MSAIMH
Acknowledgements : Bionic Research Institute - Chute Design Conference 1992
THE SUCCESS of belt cleaning is largely affected by chute design.
Adequate provision must be made at the design stage for access and maintenance.
Brian Merchant is the Managing Member of Scorpio Scrapers CC. He attained the Honours Certificate of the Association of Mining, Electrical and Mechanical Engineers, and is a member of the South African Institute of Materials Handling.
THE IMPACT OF CHUTE DESIGN ON BELT CLEANING EQUIPMENT
Many items under discussion in today's seminar are directly or indirectly related to the successful operation of a belt cleaning system. Delegates will be familiar with the volumes of materials conveyed on major coal and gold mines, power stations and coal terminals and of the consequences and compounded wastage incurred when a cleaning system becomes ineffective.
Take the example of a coal conveyor traveling at a speed of 3.5m per second and carrying 3000 tons per hour with a moisture content of 8%. If 0,05% of the material conveyed adhered to the conveyor belt at the discharge point, and was carried back on the return belt, the carry back amounting to 1,5 tons per hour, would be deposited along the length of the conveyor belt.
One can readily see that in a short space of time, the carry-back could seriously inhibit the operation of the conveyor.
It therefore follows that where carry-back can be minimised, at discharge points, substantial costs savings can be achieved by reducing potential damage to belts and idlers and avoiding clean-up costs.
Apart from mechanical or financial implications, there is the safety factor when transporting combustible materials. The accumulation of carry-back materials of this nature can create hazardous situations through friction, spark or spontaneous combustion, with the potential of reducing surrounding plant to ruin in a matter of minutes.
Materials conveyed throughout the country vary in consistency from dry powders to wet slurries. Types of materials range from phosphates, coal, and iron ores, to chrome ores with a corresponding variance in densities. One can readily see that theses materials each have their own unique properties in relation to belt cleaning and great consideration has to be given to the types of belt cleaners employed.
Polyurethane for example, would be reasonably efficient on some types of coal applications but would have an extremely limited life-span on a conveyor belt carrying chrome ore.
It is only in recent years that belt cleaners have been incorporated as a necessary part of the total conveyor system. Prior to this, little if no attention was paid to the inclusion of belt scrapers at the design stage, thus necessitating modifications on-site to enable belt scrapers to be installed.
Scrapers were installed at the most convenient position but, not necessarily the most effective one.
They were installed either outside the head chute, where carry-back was not returned to the materials handling path or, inside the head chute, making access for maintenance purposes extremely difficult.
Scrapers mounted inside the head chute soon became ineffectual due to clogging or simply through lack of maintenance. Where belt scrapers were mounted outside the head chute, carry-back back was removed but not returned to the materials handling path which necessitated having to re-handle this material.
As scraper manufacturers, Scorpio believe that new chute design should address the following items :
1. Suitable openings for the correct number of belt scrapers should be allowed for at the design and manufacturing stages.
This is especially important where wear liners are included in the design, as it is then extremely difficult to cut adequate openings on-site after wear liners have been installed. The same applies to stainless steel chutes. In either case, the process of cutting chute openings becomes a lengthy one involving special equipment and, it is doubtful if the same quality of cut could be achieved compared to a manufacturing facility.
2. Where chutes are manufactured in two halves with bolt together flanges.
adequate provision must be made for clearance between the bolt-together flanges and scraper mounting brackets, as again, unsightly cutting would have to be carried out on-site.
3. Ideally, chute sides should remain parallel to the full depth of the scraper mounting bracket, before angling in.
This will reduce any possible spillage through the chute openings. Should this not be feasible, due to reduced clearances between conveyors, a gusset should be provided to secure the lower end of the mounting bracket and covers for openings should be provided for.
4. The angle of head chute and dribble chute back plates should be steep enough to allow scraped material to flow down to avoid any possible build-up. The design should also allow sufficient clearance between the scraper support tube and the back plate
to avoid any possibility of a bridging effect resulting in a build-up of scraped material fouling or lifting the belt.
5. The installation position of scrapers should be considered when designing structural steel for the support of chutes and end pulleys.
Correct positioning of steelwork simplifies initial installation procedures and allows easy access for regular maintenance to be carried out efficiently. Restricted access to belt scrapers can either compound maintenance costs over the long term or, result in a complete lack of maintenance.
6. The height of walkways alongside conveyors can be another factor governing easy access for maintenance purposes.
Should the walkway be at the same level as the belt scraper, difficulty will be experienced with removal and re-installation. One way of alleviating this problem would be to step down the walkway adjacent to the scraper thereby allowing easy access to belt scrapers.
7. Sufficient clearance should be allowed between the edge of the pulley and chute side plates, to accommodate minor variances in belt alignment. Should the clearance be insufficient, there is a great likelihood that the edge of the belt can come in contact with the chute side plates and fold over. This could, in turn, lead to subsequent damage to the wearing parts of the scraper.
8. When a conveyor head pulley has a manual or automatic adjustment such as a screw take-up (this is especially common on stackers and reclaimers) the correct installation position of scrapers can easily be affected by any movement of the head pulley. This can be overcome by belt scrapers being mounted onto an adapter plate, which in turn, is attached to the adjustment mechanism. This method of mounting would allow the scraper to retain its relationship with the head pulley thereby avoiding any possibility of scraper and belt damage.
9. Support members for 'blocked chute' or 'bin full' detectors should always be positioned well clear of scraper support tubes
Sufficient clearance will avoid the possibility of scraped material bridging across the two, causing an accumulation. This is especially important when materials with a high moisture content are being conveyed.
Belt conveyors are relatively simple pieces of equipment if good design principles are applied. Nightmares for maintenance staff can occur however, if the required amount of fore-thought is not given to the design. Belt scrapers in turn, are one of the few necessary items on a conveyor belt that require regular maintenance. Is it not therefore logical to assume, that the design should allow for this?